Why Does My Aircon Leak Only When It Rains?
A leak that only appears during rain is rarely a standard drain fault. It usually means rainwater is entering through a wall opening, weather pressure is disrupting drainage, or water is reaching electrical points, and each scenario needs a different response.
1. External Rain Ingress Through Wall Or Trunking Path
How This Works
Every wall-mounted aircon installation requires a penetration through the exterior wall. That is the pipe sleeve through which the refrigerant lines, drain pipe, and electrical cable pass. When this penetration is not properly sealed with waterproof compound, rainwater can enter the gap and track along the pipe sleeve into the interior wall cavity. The same happens when the sealant degrades after a few years of sun and rain. From there the water follows gravity to the lowest accessible point. It often appears at the trunking edge, the indoor unit frame, or a wall seam well away from the original entry point.
How To Tell
Rain ingress through a wall opening produces leaking that tracks directly with rainfall. The unit can be completely switched off and the drip still appears. Drain backflow needs the unit to be running plus a specific wind-pressure pattern at the outlet. This path is purely weather-driven and repeats on any rain event. The electrical risk path escalates quickly, with breaker trips or odors. Rain ingress here usually shows as clean water along a wall seam or trunking edge. Trace the trail back toward the wall penetration and check whether the pipe sleeve sealant is intact.
- Leak appears during rain and reduces after weather clears.
- Water trails follow wall opening or trunking route.
- Cooling may still feel normal.
How We'd Confirm It
We trace the entry path from outside, seal the wall penetration or trunking gap, and confirm the indoor leak stops after the next rain event.
Gas or cooling work will not fix a rain ingress path.
2. Drain Path Affected By Weather Pressure Pattern
How This Works
The condensate drain pipe exits to the exterior of the building at a point exposed to outdoor air. During heavy rain with strong wind, common in Singapore's squall-line storms, sustained pressure builds at the drain outlet. That pressure can push air, and in severe cases condensate water, back through the pipe. The backflow collects at the lowest indoor point, usually the drain tray. Once the tray fills, it overflows through the same path as a standard drain blockage.
How To Tell
Drain backflow only occurs when the unit is actively running during rain with strong wind. It needs both conditions at once. Direct rain ingress happens whether the unit is on or off. Backflow is operation-dependent and tied to specific squall-line weather patterns, not every rainstorm. There is no breaker trip or burning smell like the electrical risk path. The drip comes from the same path as a regular drain leak. That is why weather dependency is the key diagnostic question: note whether the leak stops immediately when the unit is switched off during a storm.
- Leak appears while unit runs during rain periods.
- Water pattern changes with wind and rain intensity.
- Drip points shift between outlet and nearby paths.
How We'd Confirm It
We inspect the drain outlet exposure and routing, add a wind-guard or re-route the discharge point if back-pressure is confirmed, and retest during operation.
Treating this as a permanent indoor leak without weather context can mislead diagnosis.
3. Rainwater Near Electrical Points
How This Works
When rainwater enters through a wall penetration or tracks along the pipe sleeve, it does not always take the most visible path. It can run inside the trunking, along the wiring conduit, or between the wall lining and the structure before becoming visible. The aircon isolator switch, the terminal block inside the indoor unit, and the PCB wiring harness are all potential interception points. Any of them is a serious shock and fire risk if water contacts live terminals under load.
How To Tell
What sets this path apart from ingress and drain backflow is electrical symptoms. A breaker that trips during rain, a burning or plastic smell near the indoor unit, or visible moisture at the isolator switch or wiring conduit. With the other two paths, the water is cosmetically damaging but not immediately dangerous. Here rainwater has reached live components. The unit can appear to operate normally between rain events. That is what makes this the most easily underestimated of the three patterns.
- Water appears near switches, isolator, or wiring points.
- Breaker trips during rain-linked leak events.
- Electrical smell appears with leak pattern.
How We'd Confirm It
Stop using the unit until electrical safety is confirmed. We isolate ingress and verify safe power condition first.
Turn the circuit breaker off and do not attempt a restart. Do not operate the unit while water is near electrical points, even briefly to test whether it trips again.
Useful Next Steps
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